The field of the present disclosure relates to optical waveguides. In particular, apparatus and methods are described herein that employ a waveguide tap or splitter on a waveguide substrate for attenuating the output of a light source.
A common configuration for an optoelectronic device includes a substrate on which are formed one or more optical waveguides, and at least one light source positioned (perhaps mounted on the substrate) to launch at least a portion of its optical output signal into an optical waveguide on the substrate. The optical signal thus launched propagates along the optical waveguide in a corresponding guided optical mode that is substantially confined in two transverse dimensions.
In many instances, a standard light source (e.g., a laser diode) is incorporated into the assembled optoelectronic device; the standard light source might be manufactured by the same manufacturer that assembles the optoelectronic device, or might be obtained from a different manufacturer of merchant or OEM light sources. In some cases, the optimum operating output power of the standard laser diode is larger than the maximum optical signal power permitted or desired in or from the optoelectronic device (e.g., to comply with an established industry standard). Operating the laser diode at reduced output power, by reducing the drive current to a level that is not sufficiently above its lasing threshold current, can reduce the maximum speed or frequency at which the laser output can be modulated, or can reduce the risetime or introduce timing jitter at the leading edge of a modulated waveform. Operating at reduced current might also introduce spectral changes, power fluctuations, or other undesirable fluctuations or instabilities, or might require more precise control of DC laser bias current or modulation current amplitude to maintain a fixed extinction ratio.
Redesigning the laser diode to run optimally at lower output power, or re-sourcing a merchant laser diode to replace it with a different one that operates at lower output power, can incur significant costs, risks, and penalties, both technical and commercial. A more straightforward approach might include intentional introduction of an optical loss element into the optical waveguide to reduce the power level of the propagating optical signal after it leaves the laser diode. In that way the standard laser diode or other light source can be operated at its optimal power level, but only a desired fraction of that output power propagates beyond the optical loss element.